1. Field of the Invention
The present invention relates to semiconductor devices having improved distortion characteristics.
2. Background Art
The trend toward digitization of communications systems has created a great need to improve the distortion characteristics of the semiconductor devices in the output stage of communication devices, as well as to reduce the size and increase the output power and efficiency of these semiconductor devices.
When two signals of different frequencies f1 and f2 (fundamental frequencies) are input to a semiconductor device, second harmonics having frequencies of f1×2 and f2×2 are typically generated and mixed with the fundamental frequencies, forming additional signals, or distortion components, at frequencies of 2×f1−f2 and 2×f2−f1, which are very close to the fundamental frequencies. This type of intermodulation distortion is referred to as “third order intermodulation distortion,” or “IMD3,” and caused by nonlinear characteristics of the semiconductor device. Such intermodulation distortion may cause noise between adjacent lines. To prevent this, a communications system employing a plurality of communication lines (or frequency multiplex communication) requires semiconductor devices having low distortion characteristics.
The relationship between the distortion characteristics of a semiconductor device and electrical parameters thereof may be analyzed using Volterra series representation. (See, e.g., R. A. Minasian, IEEE Trans. Microwave Theory Tech., vol. 28, No. 1, pp. 1-8, 1980.) A Volterra series expansion for determining the IMD3 indicates that increasing the transconductance gm of the semiconductor device or reducing its second derivative gm″ (with respect to the bias voltage) may be effective in improving the distortion characteristics of the semiconductor device.
An effective way (theoretically proven) to reduce the second derivative (gm″) of the transconductance is to establish a doping profile in the channel layer of the semiconductor device such that the dopant impurity concentration varies inversely with the third power of depth, x, measured into the layer from the surface (that is, the dopant impurity concentration is proportional to x−3). (See, e.g., R. A. Pucel, Electronics Lett., vol. 14, No. 6, pp. 204-206, 1978.) We fabricated such a semiconductor device and investigated its distortion characteristics, as described below.
FIG. 9 is a cross-sectional view of a conventional semiconductor device. Referring to FIG. 9, the following layers are sequentially formed on top of one another on a semi-insulating GaAs substrate 11: an undoped AlGaAs/undoped GaAs superlattice buffer layer 12, an undoped GaAs buffer layer 13, an n-type GaAs channel layer 101, an n-type AlGaAs Schottky junction forming layer 15, an n-type GaAs lower contact layer 16, an n-type AlGaAs etch stopper layer 17, and an n+-type GaAs upper contact layer 18.
A source electrode 19 and a drain electrode 20 are formed on the n+-type GaAs upper contact layer 18. A recess structure is formed through the n-type AlGaAs etch stopper layer 17 and the n+-type GaAs upper contact layer 18. A gate electrode 21 is disposed within the recess structure between the source electrode 19 and the drain electrode 20 and forms a Schottky junction with the n-type AlGaAs Schottky junction forming layer 15.
The n-type GaAs channel layer 101 has a graded doping profile such that the dopant impurity concentration varies inversely with the third power of depth, x, measured into the layer from the surface (that is, the dopant impurity concentration is proportional to x−3). The n-type GaAs channel layer 101 has a thickness of 1800 Å, and its top surface has a dopant impurity concentration of 2.3×1017 cm−3.
FIG. 10 is a diagram showing measured distortion characteristics (namely, adjacent channel power, or ACP) of this conventional semiconductor device. FIG. 10 also shows, for comparison, measured distortion characteristics (or adjacent channel power) of a widely used semiconductor device in which the channel layer has a uniform dopant impurity concentration as a function of depth. The channel layer of the comparative semiconductor device is made of n-type GaAs and has a dopant impurity concentration of 1.5×1017 cm−3 and a thickness of 1300 Å. That is, this channel layer has substantially the same sheet dopant impurity concentration and the same pinch-off voltage as the channel layer of the semiconductor device described above and having the graded channel doping profile.
These semiconductor devices have a gate length Lg of 1.1 μm and a gate width Wg of 12.6 mm. They were each mounted in a surface mount discrete package. The distortion characteristics of each semiconductor device were measured when a 2.14 GHz W-CDMA modulated signal (3GPP TEST MODEL 1, 64 code single signal, 3.84 MHz channel bandwidth) was input to the device with the drain voltage Vd and the drain current Id set to 10 V and 300 mA.
The measurement results clearly show that the semiconductor device having the graded channel doping profile has lower distortion characteristics than the comparative semiconductor device having the uniform channel doping profile.
FIG. 11 shows measured relationships between the drain current Id and the transconductance gm of the semiconductor device having the graded channel doping profile and between the drain current Id and the second derivative gm″ of the transconductance gm (with respect to the gate bias). FIG. 11 also shows the same relationships for the comparative semiconductor device having the uniform channel doping profile.
As shown in FIG. 11, these semiconductor devices have substantially the same transconductance (gm). However, the transconductance gm of the semiconductor device having the graded channel doping profile has a smaller second derivative (with respect to the gate bias) than that of the comparative semiconductor device having the uniform channel doping profile. From this, it may be concluded that the improved distortion characteristics of the semiconductor device having the graded channel doping profile results from the fact that its transconductance gm has a reduced second derivative gm″ (with respect to the gate bias).
However, generally the characteristics of a field-effect semiconductor device tend to be affected by its surface state, as well as by the state of its buffer layer side. This means that it is difficult to achieve ideal transconductance characteristics (that is, transconductance characteristics whose second derivative gm″ with respect to the gate bias is zero) by the creation of the above channel doping profile alone.